For applications of suppressing a power generation fluctuation which utilizes natural energy like solar light and wind power, suppressing a power demand fluctuation, and peak shifting, the utilization of a large-scale secondary battery system including secondary batteries is expected. In particular, a large-scale secondary battery system that utilizes lithium-ion batteries which have a remarkably improved performance is becoming available in recent years. Regarding such large-scale secondary battery system, a long-term operation, such as 15 years or 20 years, is expected.
Regarding such large-scale battery system, a cell which is a minimum unit of the secondary battery in single or plurality connected in parallel are connected in series, and may further be connected in parallel. Hence, a series connection configuration and a parallel connection configuration are typically repeated hierarchically to construct the system. For a replacement unit of when malfunctioned or deteriorated, it is not general for the large-scale secondary battery system that utilizes lithium-ion batteries to make each single cell replaceable, but are a block, which is so-called a module or a unit, including several to dozens of cells In this specification, the minimum unit replacement will be referred to as a battery module.